Only one.

You open the one labeled peaches. It cannot contain peaches. There are two cases, either it contains pears, or plums.

1. You found plums in the jar labeled peaches. Since the remaining jar labeled pears can't contain the pears, the pears must be in the jar labeled plums.
2. You found pears in the jar labeled peaches. Since the remaining jar labeled plums can't contain plums, the plums must be in the jar labeled pears.